Wustite bed improvement

ABSTRACT

In the fluidized reduction of specular hematites, the process in the wustite reducing bed is terminated prior to the formation of an equilibrium wustite composition, thereby providing for improved reducibility of the ore to metallic iron in subsequent reducing beds.

ite States Patent 1 Draeger 51 Jan. 9, 1973 JWUSTITE BED IMPROVEMENT [21] Appl. No.: 47,648

[52] U.S. Cl. ..-.75/26, 75/34, 75/1 [51] Int. Cl. ..C22b 1/10, C2lb 13/00 [58] Field of Search ..L ..75/26, 9, 34

[56] References Cited UNITED STATESPATENTS 2,921,848 l/l960 .Agarwal ..75/26 3,479,232 Broussard ..75/34 3,317,308 5/1967 Greffe 3,364,010 1/1968 Mayer 3,511,642 5/1970 Etherington....

3,374,087 4/1968 Gray Primary ExaminerWinston A. Douglas Assistant Examiner-Peter D. Rosenberg 'Attorney'-Manahan and Wohlers and Joseph J.

Dvorak [57] ABSTRACT In the fluidized reduction of specular hematites, the process in the wustite reducing bed is terminated prior to the formation of an equilibrium wustite composition, thereby providing for improved reducibility of the ore to metallic iron in subsequent reducing beds.

8 Claims, 2 Drawing Figures PAIENTEIMI 9 W FIGURE I PHASE DIAGRAM FOR MAGNETITE -WUSTITE-IRON SYSTEM 0.III

MAGNETITE 800 I000 I200 I400 I600 I800 2000 2200 TEMPERATURE, F.

FIGURE 2 EQUILIBRIUM RATIO OF HgO/Hg VERSUS RATIO OF COg/CO 2250F 2000F I750F. I

CO2 /co x10 Kenneth S Draeger INVENTOR BYM ATTORNEY 1 wus'rms BED IMPROVEMENT BACKGROUND OF THE INVENTION The present invention relates to an improved process for producing iron from iron ores. More particularly, the present invention relates to a method of direct iron ore reduction. Specifically, the invention relates to the direct reduction of specular hematites in a fluidized reduction process.

A number of processes have been developed to reduce iron ore in fluidized beds using reducing gases. Generally, the iron ore is depicted in these processes as proceeding, during reduction, through a series of discrete stages or stoichiometric composition of iron oxides, such asFe O Pe o and FeO. In actuality, only ferric oxide, F6 0,, is a normal stoichiometric compound. Additionally, many chemical constituents are present in iron ore. Consequently, the chemical equilibrium and reaction mechanisms of iron ore reduction processes are extremely complex.

Notwithstanding the complexity of iron ore reduction processes, from a chemical standpoint, generally the only elements of concern are iron, oxygen, hydrogen and carbon. Therefore, it has been possible to study the chemical equilibria of these reduction processes.

As a result of studies of equilibrium phenomena in fluidized iron ore reduction processes, iron ore is depicted as passing through several discrete stages or oxidation states in its. reduction-to metallic iron. For example, the'iron ore to be reduced is generally depicted as the stoichiometric compound ferric oxide, l e- During reduction the ferric oxide containing ore is considered to be reduced to magnetite or Fe O Magnetite is a material which below about l,500 F. has a stoichiometric composition but which contains oxygen as a solid solution at higher temperatures and is non-stoichiometric. I

Next, the magnetite is considered as being reduced to wustite, sometimes described as ferrous oxide, FeO.

Wustite is a non-stoichiometric compound often designated by the formula FeO,. The composition of wustite, i.e., the value of x in FeO,, at equilibrium ranges from about 1.0 to about l.33at temperatures of about l,l00 F. to about 2,600 F., depending upon the composition of the gas phase with which the solid is the contact. I

In the last step of the reduction process, wustite is depicted as being reduced to metallic iron, Fe.

In commercial practice, the stages or oxidation states through which the ore is considered to proceed generally are achieved by conducting the reduction in appropriate, separate, reducing zones, each of which may be composed of one or more fluidized beds.

Typically, iron ore is ground to the proper'particle size for fluidization by a reducing gas. The ground ore then is charged to a preheating unit where the temperature of the ore is raised to about the operating temperature of the reduction process. Next, preheated ore is charged into the fluidized bed reactor where it is brought in contact with the hot reducing gases.

Normally, the reduction is accomplished in two separate reduction zones which may be comprised of a plurality of fluidized beds. In the firstreduction zone, the ferric oxide containing ore is reduced untilan equilibrium wustite composition is reached.

Since the reducing power of any gas is effectively limited by equilibria between the unreacted reducing gases and the oxidized gases in zone or bed where the reduction takes place, the state of reduction of the ore will be a function, inter alia, of the gas equilibria. Consequently, there is a specific gas composition which corresponds to equilibrium wustite composition at a given temperature. For example, when carbon monoxide is used as the reducing agent, there will be a specific carbon dioxidezcarbon monoxide ratio which is in equilibrium with the percentage of oxygen in the iron. Therefore, whether equilibrium has been reached in the first reduction zone is determined by reference to the composition of the solids and gases at the specified temperatures in that zone.

As was stated previously, the ore is partially reduced in the first zone at least until an equilibrium wustite composition is reached. This procedure is followed for numerous reasons. For example, the rate at which metallic iron is formed depends mainly upon the rate at which wustite is reduced to iron in the ferrous reducing zone. Consequently the overall productivity theoretically is enhanced if the partially reduced ore charged in the ferric reduction'zone is completely reduced to the equilibrium wustite composition. Also, it is generally considered to be a more economical process if an efficient utilization of reducing gas is provided for by carrying out the reduction to the equilibrium composition in the wustite zone.

Therefore, as a practical matter, iron ore is charged into the ferric reduction zone or wustite bed of the fluidization reactor and held fluidized by reducing gases at process temperatures at least until an equilibrium wustite composition is reached.

The above practice is followed for any iron ore whether it be a relatively easily reduced earthy hematite ore, or a difflcultyly reduced specular hematite ore. Similarly, the above practice is essentially followed whether the wustite is intermittently or continuously withdrawn from the wustite zone.

Typically, after achieving equilibrium in the wustite bed, the partially reduced ore is advanced to the next bed or beds in the ferrous reduction zone. In the ferrous reduction zone, the wustite is reduced to a highly metallized product.

The extent of metallization, of course, refers to the amount of total iron in the reduced ore that is present as elemental iron. Generally, metallization is expressed as a percent.

SUMMARY OF THE INVENTION I ing from about 1,100 E. to about 1,800 E. In the first 7 or wustite stage of the reduction process, the reduction is terminated prior to the formation of an equilibrium wustite composition. In the subsequent or final reduction stages, the partially reduced ore is fluidized and reduced to a highly metallized product.

In the practice of my invention, reduction in the wustite stage preferably is terminated when the number of parts of oxygen combined with iron in the reduced ore is about 0.01 to about 0.20 parts greater than the number of parts of Oxygen combined with iron in equilibrium wustite.

' BRIEF DESCRIPTION OF THE DRAWING FIG. l is a phase diagram for the magnetite-wustiteiron system.

FIG. 2 is a diagram showing the equilibrium ratio of H to H versus CO to CO for various temperatures.

DETAILED DESCRIPTION OF THE-INVENTION reduced ore from the wustite stage is reduced to metallic iron.

Although more than two stages can be used in practicing this invention, I prefer to conduct my process in two stages and in at least two separate reduction beds.

Even a greater number of fluidized reduction beds may.

be employed, however. I

In accordance with this invention, a specular hematite ore is ground and dried and sized for gaseous reduction in a fluidized bed. The grinding, drying and sizing steps are well known in the art.

The ore can be dried, for example, by heating the ore in a kiln to about 300 F. Selection of proper time and temperature for drying will depend upon the allowable water content of the ore. Generally, after drying, the ore will contain less than wt. percent water and preferably less than 6 wt. percent water. Even more preferred, the ore willcontain less than 4 wt. percent water. I

Grindingcan be achieved, by subjecting the ore to ball milling, impact'or autogenous crushing and the likefSizing of the ore is accomplished generally by screening.

. Typically the ore employed in this process will have particle sizes essentially within 1 to 10,000 microns in size.. The average particle size of the ore will range generally fromabout 40 to about 500 microns in size. Preferably, the particle sizes will range from 6,000 microns in size and finer with no more than 10 wt. percent of particles below about 44 microns in size.

After drying, grinding and sizing of the ore, the pulverized material is first charged to a preheater where it is heated preferably to about the operating temperature of the wustite reducing bed. Generally the temperature of the wustite bed will be from about l,l00 F. to about l,800 F.

Advantageously, preheating will be accomplished by fluidizing the ore charge with an effluent gas from a reducing bed and combusting in the fluidized bed any unreacted reducing gases. However, the preheating rotary kiln, shaft furnace and the like.

In any event, after the temperature ofthe ore is raised, and preferably raised to about the operating temperature of the wustite reducing bed, the preheated ore is transferred to the wustite reduction zone where it is reduced and fluidized in a stream of ascending hot reducing gases.

Although there may be a number of reducing gases which can be employed in this process, I prefer to use CO-rich reducing gas. Typically the reducing gas will be a mixture of C0, C0 H and H 0. Such a reducing gas can be obtained, for example in a separate gasification zone. Optionally, a hydrocarbon such as methane and air can be injected into the ferrous reduction zone and combusted therein to produce a reducing gas as well as process heat. These methods for producing reducing gases are well known in the art and do not constitute a part of this invention.

' The reducing gas, whatever its source, is delivered into the top or wustite bed at a pressure sufficient to fluidize and maintain a proper flow of solids through the system.

The iron ore is reduced in the wustite bed preferably at a temperature of about 1,200 F. to about l,500 F. and at a pressure of about 0 to 175 psig. However, the temperature in the first stage can range more generally from about l,l00 F. to about l,800 F. and a pressure composition. Generally, however, the reducing gas wustite at equilibrium. In other words, reduction in the should have a ratio of CO :CO of about 0.2 1.5 with 0.75 l.0 being preferred. The ratio of H 0 to H can be 0.2-3.0 with the ratio of 0.5-1.0 being preferred. These ratios relate to these constituents in the off gas from this zone.

The reduction in the wustite stage or zone, in accordance with this invention, is terminated prior to the reduction of the ore to equilibrium wustite composition. Generally, the reduction in the wustite zone will be terminated when the composition of wustite is in the range of about Feo.,,,;,,, to FeO where x is the number of parts of oxygen combined with iron in wustite stage is terminated when the number of parts of oxygen combined with iron in the partially reduced ore is about 0.01 to about 0.20 parts greater than that which would be combined with equilibrium.

As stated previously, wustite is a nonstoichiometric compound often designated by the formula FeO The value of x in FeO at equilibrium is a function of temperature as well as the composition of the gas in equilibrium with the wustite. Depending upon the temperature and the gas composition, the equilibrium composition of wustite can be read from a typical phase diagram such as that shown in FIG. 1. The FIG. 1 phase diagram is based on the reduction of magnetite-with hydrogen to yield wustite and water. The FIG. 1 diagram is conveniently extended to the CO reduction iron at wustite .system and mixed gas systems through the established water gas shift equilibria. The relationship between the H 0 to H ratio and the CO to CO ratio is shown diagrmatically in FIG. 2.

Because there is a relationship between the solids equilibria and the reducing gas equilibria, the Fe:O ratio at equilibrium can be determined from an analysis of the reducing gas composition in the reducing zone. As a practical matter, the actual Fe:0 ratio is determined by analysis of a sample of solid taken from the wustite zon'e.

Reduction can be conducted on a batch or continuous basis. In a continuous process, termination of the reduction reaction in the wustite stage generally is accomplished by continuously discharging the ore into a lower zone, such as the final reduction zone. in a batch process, termination'of the wustite reduction stage can be accomplished, for example, by-introducing into the wustite stage a higher reducing power gas, thereby in effect switching immediately to final reducing stage conditions.

in a preferred embodiment of the present invention The reduction in the final two beds of the reactor is conducted to provide a product of a high degree of mctalization generally of from about 80 to 98 percent metalization and preferably of about 85 to 95 percent metalization.

Table l illustrates the advantages of terminating the reduction in the wustite bed prior to reaching equilibrium wustite. In Run A, the reduction in the wustite bed was terminated after 15 minutes in that bed and before equilibrium wustite was reached. The ore was then rapidly reduced to a relatively high metalization in the ferrous reduction zone. On the other hand, in Run B, the ore was reduced to equilibrium wustite in about minutes. After a total residence time of 120 minutes in the wustite bed the partially reduced ore of Run B was then reduced in a ferrous reduction zone. As can be seen the reduction in the subsequent zone was more difficult and at a lower reaction rate when equilibrium wustite was employed.

the solids are continually fed to and withdrawn from 20 TABLE [Two stage reduction of specular hematite ore] Reductions conditions Results X, in X, in Reduction Twig Pres, Time, FeOx, F00;- stage p.s.i.g. Gas composition minutes RunA RunB Start 1. 50 1. 50 15 1. 075 1.077 wustite. 1,320 16 62.3% Hz and 37.7% 1110.. 1.062 1.062 115 0.302 Ferrous..... 1,500 15 76% H1 and 24% E10 0. 099 0. 208 60 0.065 0. 160 120 0 042 0.105

the wustite bed so long as the average residence time Upon completion of the reduction in the ferrous for the ore in the wustite bed does not exceed 30 minutes and preferably does'not exceed 15 minutes and the value for x in FeO, is about 0.05 to about 0.20 pai'ts greater than the equilibrium value for x, with about 0.10 parts greater being preferred.

The practice of this invention is equally applicable to batch reduction systems or processes. ln such systems, the partially reduced ore in the wustite bed is held fluidized under reducing conditions for not greater than about 30 minutes and preferably not greater than 15 minutes while the ore is reduced until the amount of oxygen combined with iron is from about 0.01 to about 0.20 parts, preferably from about 0.01 to about 0.10 parts and more preferably from about 0.01 to about 0.05 parts greater than that in equilibrium wustite. Then, reduction in the wustitebed is terminated and the ore is discharged into a final reduction zone.

In any event, as soon as the reduction is terminated in the wustite bed in accordance with this invention, the partially reduced ore is reducedin a subsequent ferrous reduction zone. Generally, the reduction in the ferrous reduction zone will be conducted in two or more beds. The partially reduced ore is reduced to metallic iron in this zone at temperatures ranging from about l,200 F. to about l,900 F. and generally at preferred temperatures of about 1,300" to'about l,600 F. The ratio of C0, to CO -is controlled in this final reduction zone so that the gases are reducing with respect to iron oxide. Generally a ratio of about 0.2 0.6 is used with a ratio of 0.2 0.4 being preferred. The ratio of H 0 to H, can be 0.2 0.6. Preferably a ratio of 0.3 0.5 is maintained.

stage, reduced iron material can be recovered from the reactor and agglomerated or briquetted, for example, for storage and prior to ultimate use in steel making processes and the like.

Having described my invention, what 1 claim and seek to be secured by Letters Patent is:

1. in a direct reduction processwherein a difficult to reduce ore is fluidized by reducing gases and partially reduced to an equilibrium wustite composition in a first reduction zone and the partially reduced ore subsequently is reduced to a highly metallized product in a ferrous reduction zone, the improvement comprising terminating the reduction of the ore in the first reduction zone when the number of parts of oxygen combined with iron in that zone is about 0.01 to about 0.20 parts greater than the oxygen combined with iron in equilibrium wustite thereby enhancing the rate of reduction of the partially reduced ore in the ferrous reduction zone.

2. The process of claim 1 wherein the reduction in the first zone is terminated by continuously discharging the partially reduced ore into a ferrous reduction zone when the number of parts of oxygen combined with iron in the first zone is about 0.05 to about 0.20 parts greater than the oxygen combined with iron in equilibrium wustite.

3. The process of claim 1 wherein the difficult to reduce ore is a specular hematite.

4. The process of claim 1 wherein the temperature in the first zone is from about l,l00 F. to about l,800 F. and the temperature in the ferrous zone is from about l,200 F. to about 1,900 F.

5. A fluidized process for reducing difficult to reduce particulate oxidic ore comprising contacting and fluidizing said particulate ore with a reducing gas in a wustite reduction zone whereby said ore is partially reduced, terminating contacting of said ore with said reducing gasin the wustite reduction zone when the number of parts of oxygen combined with iron in said ore is from 0.01 to 0.20 parts greater than oxygen in equilibrium wustite, withdrawing a partially reduced ore from a wustite reduction zone and subsequently reducing said partially reduced ore to a highly metalized product in a ferrous reduction zone.

reduce ore is a specular hematite. 

2. The process of claim 1 wherein the reduction in the first zone is terminated by continuously discharging the partially reduced ore into a ferrous reduction zone when the number of parts of oxygen combined with iron in the first zone is about 0.05 to about 0.20 parts greater than the oxygen combined with iron in equilibrium wustite.
 3. The process of claim 1 wherein the difficult to reduce ore is a specular hematite.
 4. The process of claim 1 wherein the temperature in the first zone is from about 1,100* F. to about 1,800* F. and the temperature in the ferrous zoNe is from about 1,200* F. to about 1,900* F.
 5. A fluidized process for reducing difficult to reduce particulate oxidic ore comprising contacting and fluidizing said particulate ore with a reducing gas in a wustite reduction zone whereby said ore is partially reduced, terminating contacting of said ore with said reducing gas in the wustite reduction zone when the number of parts of oxygen combined with iron in said ore is from 0.01 to 0.20 parts greater than oxygen in equilibrium wustite, withdrawing a partially reduced ore from a wustite reduction zone and subsequently reducing said partially reduced ore to a highly metalized product in a ferrous reduction zone.
 6. The process of claim 5 wherein said contacting of the ore with a reducing gas does not exceed 30 minutes at temperatures ranging from about 1,100* F. to about 1,800* F.
 7. The process of claim 5 wherein said contacting is terminated by continuously withdrawing partially reduced ore from the wustite zone when the number of parts of oxygen combined with iron in the withdrawn partially reduced ore is about 0.10 parts greater than the oxygen combined with iron in equilibrium wustite.
 8. The process of claim 5 wherein the difficult to reduce ore is a specular hematite. 